Small-molecule inhibitors of Rac1 in metastatic cancer cells

ABSTRACT

A novel inhibitor of Rac activity based on the structure of the established Rac/Rac-GEF inhibitor NSC23766 is discloses. The compound EHop-016, with an IC50 of 1.1 μM, is a 100-fold more efficient inhibitor of Rac activity than NSC23766. EHop-016 is specific for Rac1 and Rac3 at concentrations ≦5 mM. At higher concentrations, EHop-016 inhibits the close homolog Cdc42. In MDA-MB-435 cells, EHop-016 (≦5 mM) inhibits the association of the Rac-GEF Vav2 with a nucleotide-free Rac1(G15A), which has a high affinity for activated GEFs. EHop-016 does not affect the association of the Rac-GEF Tiam-1 with Rac1(G15A) at similar concentrations. EHop-016 also inhibits the Rac activity of MDA-MB-231 metastatic breast cancer cells and reduces Rac-directed lamellipodia formation in both cell lines. EHop-016 decreases Rac-downstream effects of p21-activated kinase (PAK)1 activity and directed migration of metastatic cancer cells. At low concentrations (&lt;5 μM), EHop-016 does not affect cell viability.

GOVERNMENT INTEREST

The claimed invention was made with U.S. Government support under grantnumber BCRP W81XWH-07-1-0330 awarded by the Department of Defense(DoD)/US Army and grant numbers SC3GM084824, G12RR035051, 5U54CA096297,and G12RR03035 from the National Institutes of Health (NIH). Thegovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

Rho family GTPases (Rho, Rac, Cdc42) are important intracellularsignaling proteins that control diverse cellular functions, includingactin cytoskeletal organization, invasion and metastasis,transcriptional regulation, cell cycle progression, apoptosis, vesicletrafficking, and cell-to-cell and cell-to-extracellular matrixadhesions. Consequently, Rho GTPases have been implicated in cancer, andthe progression of other diseases by a large number of studies. Of theRho family GTPases, Rac1 and Rac3, the isoforms expressed innon-hematopoietic cells, have been specifically implicated inrearrangement of the actin cytoskeleton into cell surface protrusionscalled lamellipodia or invadopodia that are specific for forwardmigration during invasion; and thus, have been implicated in promotionof metastasis. Racs have also been shown to be essential for Ras andother oncogene-mediated transformation. Our group and others as well,has implicated hyperactive Rac1 and Rac3 with increased survival,proliferation, and invasion of breast and brain cancers. Recent reportshave shown a role for Rac in mammalian target of rapamycin(mTOR)-mediated regulation of cancer malignancy and anti breast cancertherapy resistance. Moreover, Rac1 was shown to increase Estrogenreceptor-alpha (ERα)-mediated transcriptional activity in breast cancer.Studies have also demonstrated a cancer-promoting role for theconstitutively active Rac1b splice variant that is overexpressed inbreast and colorectal cancer. Since the malignant phenotype of Rac isassociated with activation of its direct downstream effectorsp21-activated kinases (PAKs), much effort has been focused on thedevelopment of PAK inhibitors as anti cancer therapeutics. However, inaddition to PAK, Racs have multiple downstream effectors such as WAVEand Mena/VASP that contribute to cancer. Therefore, targeting Rac is amore viable approach for the development of anticancer drugs.

Unlike the related small GTPase Ras, Racs are not mutated in malignantcancers but rather overexpressed or hyperactivated. Racs are activatedby GTP/GDP exchange catalyzed by guanine nucleotide exchange factors(GEF) that are regulated via a myriad of cell surface receptors. So far,over 60 potential Rac-GEFs have been identified. Of these, Dbl familyGEFs such as T-cell invasion and metastasis gene product (Tiam-1) andVav have been implicated in breast cancer progression. Tiam-1 expressionhas also been correlated with high breast cancer cell migration,indicating a specific role for Tiam-1 in breast cancer metastasis.Recent reports have also shown that PIP₃-dependent Rac exchanger 1(p-Rex1) is upregulated in breast cancer cells and breast cancerpatients with poor prognosis. Thus, elevation of Rac.GEF expressionand/or activity appears to be a common phenomenon during cancerprogression. Therefore, targeting the binding of Rac to GEFs is arational strategy to inhibit Rac activity and thus, cancer invasion.

NSC23766 was identified as a small molecule that binds to a putativebinding pocket in the surface groove of Rac1 that interacts with theRac-specific GEFs Trio and Tiam1. NSC23766 has been shown to inhibit theanchorage-independent growth and invasion of human prostate cancer PC-3cells, Rac activation and Rac-dependent aggregation of plateletsstimulated by thrombin, Rac1 and Rac2 activities of hematopoieticstem/progenitor cells, and migration from mouse bone marrow toperipheral blood. NSC23766 has also been shown to inhibit invasion ofchronic myelogenous leukemia cells in vitro and in vivo in a mousemodel. Thus, such structure-function based rational design appears torepresent a new avenue for generating small molecule inhibitors of Rac.However, NSC23766 is a moderately active Rac inhibitor with a relativelyhigh IC₅₀ of 50-100 μM in fibroblasts, which limits its potential use asa therapeutic agent. In addition, our group has found that in the highlymetastatic cancer cell line MDA-MB-435, NSC23766 inhibits Rac1 by only˜20% at a concentration of 50 μM, and that at this concentration thereis no significant effect on lamellopodia formation. Therefore, there isa need for more effective inhibitors to reduce Rac activity in highlymetastatic cancer cells.

The identification of novel inhibitors of Rac that function viadifferent inhibitory mechanisms has been the subject of several studies.Thus, whereas NSC23766 inhibits the interaction of Rac1 with several ofits GEFs, the Rac inhibitor EHT 1864 interferes with the interaction ofRac with its downstream effectors at concentrations of 10-50 μM. Avirtual screening of a selected subset of compounds from the ZINCdatabase for binding affinity to Rac1 based on the crystal structure ofRac1 with NSC23766, identified several novel Rac1 inhibitors withexperimental IC₅₀ values ranging from 12.2-57 μM. In addition, ahigh-throughput flow-cytometry bead-based multiplex assay identifiedMLS000532223 as a compound that prevents GTP binding to Rac. However,other Rho GTPases, such as Cdc42, are also affected by this compound.Small molecule compounds have also been synthesized to specificallyinhibit the Rac1b splice variant. Another report identified ITX3 as aGEF inhibitor that targeted Rac and RhoG interaction with Trio; however,this compound is effective at high 50-100 mM concentrations.

SUMMARY OF THE INVENTION

In an endeavor to develop novel more potent Rac inhibitors with possibleclinical applications, the present invention uses NSC23766 as a leadstructure for the design of compounds with 2-times enhanced potency.Thus, the present invention reports the identification andcharacterization of the biological activity of EHop-016, a novelNSC23766 derivative that inhibits Rac1 100-fold more effectively thanthe parent compound.

The compound EHop-016, with an IC50 of 1.1 μM, is a 100-fold moreefficient inhibitor of Rac activity than NSC23766. EHop-016 is specificfor Rac1 and Rac3 at concentrations ≦5 mM. At higher concentrations,EHop-016 inhibits the close homolog Cdc42. In MDA-MB-435 cells, EHop-016(≦5 mM) inhibits the association of the Rac-GEF Vav2 with anucleotide-free Rac1(G15A), which has a high affinity for activatedGEFs. EHop-016 does not affect the association of the Rac-GEF Tiam-1with Rac1(G15A) at similar concentrations. EHop-016 also inhibits theRac activity of MDA-MB-231 metastatic breast cancer cells and reducesRac-directed lamellipodia formation in both cell lines. EHop-016decreases Rac-downstream effects of p21-activated kinase (PAK)1 activityand directed migration of metastatic cancer cells. At low concentrations(<5 μM), EHop-016 does not affect cell viability.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the following detailed description taken in conjunction with theaccompanying figures showing illustrative embodiments of the invention,in which:

FIG. 1A shows a synthetic scheme for the preparation of EHop-016according to the present invention.

FIG. 1B shows EHop-016 docked into GEF binding pocket of Rac1, and itscomparison with the position of NSC23766 in the crystal structure of theRac1-NSC23766 complex according to the present invention.

FIG. 2 shows a plot indicating the inhibitory effect of EHop-016 andNSC-23766 on Rac activity according to the present invention.

FIG. 3A shows a representative Western blot of pull-downs immunostainedfor Rac3-GTP (top row) or total Rac3 in cell lysate (bottom row) (n=3)according to the present invention.

FIG. 3B shows representative Western blots of pull-downs immunostainedfor Cdc42-GTP (top row) or total Cdc42 in cell lysate (bottom row) (n=2)according to the present invention.

FIG. 3C shows representative Western blots of pull-downs immunostainedfor RhoA-GTP (top row) or totalRhoAin cell lysate (bottom row) (n=3)according to the present invention.

FIG. 4A shows a representative Western blot of GST-Rac1(G15A) pull-down,supernatant (sup) from the incubation, and cell lysates, immunostainedfor Trio, Tiam-1, or Vav2 according to the present invention.

FIG. 4B shows a representative Western blot of GST-Rac1(G15A) beadspreincubated with vehicle (0), or 2 or 4_M EHop-016 prior to incubationwith MDA-MB-435 cell lysates according to the present invention.

FIG. 4C shows quantification of the percentage of average Vav2 (twobands at _(—)˜100 kDa) associated with the Rac1(G15A) beads frompull-down assays in the presence or absence of EHop-016 according to thepresent invention.

FIG. 4D shows a representative Western blot of GST Rac1(G15A) pull-downs(Top), supernatants (Bottom) according to the present invention.

FIG. 4E shows quantification of percentage of Tiam-1 (DH/PH) domainassociated with Rac1(G15A) beads in the presence or absence of EHop-016or NSC23766 according to the present invention.

FIG. 5A shows representative Western blots (MDA-MB-231; MDA-MB-435) ofpull-downs immunostained for Rac-GTP (top row) or total Rac in celllysate (bottom row) according to the present invention.

FIG. 5B shows representative micrographs for MDA-MB-231 and MDA-MB-435and the percentage of cells that demonstrated lamellipodia wasquantified for each treatment from 10 representative microscopic fieldsaccording to the present invention.

FIG. 6A shows a representative Western blot for MDA-MB-435 cells treatedwith vehicle (0) or 2 or 4 μM EHop-016 for 24 h according to the presentinvention.

FIG. 6B shows quantification of positive bands from Western blots (65kDa) using ImageJ software according to the present invention.

FIG. 6C shows representative micrographs of propidium iodide-stainedcells for each treatment at ×200 magnification and the percentage ofcells that migrated to the underside of a membrane with 8-μm diameterpores, relative to vehicle (100%) according to the present invention.

FIG. 7 shows the effect of EHop-016 on cell viability of metastaticbreast cancer cells and mammary epithelial cells according to thepresent invention.

FIG. 8A shows an alternative docking conformations of EHop-016 into theputative GEF binding pocket of Rac1 with lowest energy conformation ofcluster of size 13 and energy of −7.61 kcal/mol according to the presentinvention.

FIG. 8B shows an alternative docking conformations of EHop-016 into theputative GEF binding pocket of Rac1 with lowest energy conformation ofcluster of size 21 and energy of −7.41 kcal/mol according to the presentinvention.

FIG. 9 shows the effect of EHop-016 on Rac/Tiam-1 binding domain (DH/PH)interaction according to the present invention.

FIG. 10 shows the weight change of mice following vehicle or EHop-016administration according to the present invention.

Throughout the figures, the same reference numbers and characters,unless otherwise stated, are used to denote like elements, components,portions or features of the illustrated embodiments. The subjectinvention will be described in detail in conjunction with theaccompanying figures, in view of the illustrative embodiments.

DETAILED DESCRIPTION OF THE INVENTION Experimental Procedures

Synthesis of EHop-016

¹H and ¹³C NMR spectra were recorded on a Bruker 400 MHz Spectrometer.Mass spectra were obtained on a Hewlett Packard 6890N GC/MSSpectrometer. All chemicals were purchased from Sigma Aldrich ChemicalCompany. The synthesis of EHop-016 (5) was performed in two stepsaccording to the reaction scheme provided in FIG. 1(A), and carried outanalogous to the procedure described in (58).(2-Chloro-pyrimidin-4-yl)-(9-ethyl-9H-carbazol-3-yl)-amine 3 wasobtained as a pure compound in a yield of 53%. The product wasidentified with TLC, NMR and GC/MS. R_(f)=0.23 (3:1, Hexane-EthylAcetate); ¹H NMR (DMSO-d₆, 400 MHz) δ 1.32 (t, J=6.9 Hz, 3H), 4.45 (q,J=6.6 Hz, 2H), 6.72 (s, 1H), 7.20 (t, J=7.36 Hz, 1H), 7.47 (t, J=7.30Hz, 1H), 7.56 (s, 1H), 7.62 (t, J=8.68 Hz, 1H), 8.11 (t, J=7.36 Hz, 1H),8.27 (s, 1H), 10.1 (s, 1H); ¹³C (DMSO-d₆, 100 MHz) δ 13.7, 37.0, 109.2,109.4, 115.0, 118.7, 120.3, 121.3, 121.9, 122.3, 125.9, 129.9, 136.9,140.0, 156.9, 159.6, 162.4; LRGC-MS m/z (rel %): [M]⁺ 276 (100), [M-Cl]⁺241 (40), [M-C₅H₅N₃Cl]⁺ 134 (26).N⁴-(9-Ethyl-9H-carbazol-3-yl)-N2-(3-morpholin-4-yl-propyl)-pyrimidine-2,4-diamine5 (EHop-016) was obtained as a pure compound in a yield of 93%. Theproduct was identified to be essentially pure by TLC and NMR: R_(f)=0.34(9:1, CH₂Cl₂-MeOH); ¹H NMR (DMSO-d₆, 400 MHz) δ 1.31 (t, J=7.0 Hz, 3H),1.73 (m, 2H), 2.32 (m, 2H), 2.34 (t, J=6.89 Hz, 8H), 3.52 (m, 2H), 4.42(q, J=7.0 Hz, 2H), 5.98 (d, J=5.7 Hz, 1H), 6.69 (t, J=5.3 Hz, 1H), 7.16(t, J=7.4, 1H), 7.43 (t, J=7.2 Hz, 1H), 7.53 (t, J=9.0 Hz, 4H), 7.81 (d,J=5.4 Hz, 1H), 8.10 (s, 1H), 8.66 (s, 1H), 9.1 (s, 1H); ¹³C (DMSO-d₆,100 MHz) δ 13.7, 26.2, 36.9, 53.4, 56.3, 66.2, 108.9, 109.0, 118.2,119.7, 120.2, 122.0, 122.2, 125.6, 132.5, 135.5, 139.9, 159.8, 160.9,162.1.

Docking of Ehop-16 into the Crystal Structure of Rac1

For molecular docking, Autodock 4.0 with AutodockTools 1.5.4 as thegraphical user interface was used. The coordinates of the crystalstructure from the Rac1-NSC23766 complex were obtained. EHop-016 wasdrawn using ChemDraw Ultra 7.0 and energy minimized with MOPAC AM1 inChem3D Ultra 7.0. After removing NSC23766 from the crystal structure,AutodockTools was used to prepare the receptor and EHop-016 for docking,and to create a grid of 60 Å×60 Å×60 Å with a grid spacing of 0.375 Åcentered on the original position of NSC23766. A flexible docking of 100GA-runs was performed with the number of individuals in population setto 200 and the maximum number of energy evaluations set to 25,000,000,with other parameters accepted as suggested by AutodockTools, which alsowas used for clustering (RMS=2 Å) of the results obtained.

Rac Activity Assays

Rac activity was determined from lysates of the MDA-MB-435 humanmetastatic cancer cell line (from ATCCC) that our group has previouslyshown to have high endogenous Rac activity. MDA-MB-435 cells in culturemedia were treated with vehicle (0.1% DMSO), or varying concentrationsof Ehop-16 (0-10 μM) for 24 h. Rac1 activity was determined aspreviously described by our group, using the G-LISA Rac1 ActivationAssay kit (Cytoskeleton, Inc., Denver, Colo.).

For generation of IC₅₀ curves for each inhibitor (EHop-016 or NSC-23766)data from three independent duplicate experiments was pooled and fourparameter dose-response curves were fitted using the non-linearregression function of GraphPad Prism®.

Rho GTPase Activity Assays

Rho, Rac, and Cdc42 activities were analyzed from MDA-MB-435 celllysates by pulldown assays using GST-Rho binding domain from Rhotekin toisolate active GTP bound Rho, and a GST-p21 binding domain of PAK1 toisolate active Rac.GTP or Cdc42.GTP. Active and total Rho GTPases wereidentified by western blotting with specific antibodies: anti RhoA, antiRac (1,2,3), anti Rac3, or anti Cdc42 (Cell Signaling Technology, Inc.,Danvers, Mass.).

Cell Viability Assays

MDA-MB-435 or MCF-10 mammary epithelial cells (from ATCC) were incubatedin vehicle (0.1% DMSO) or varying concentrations of EHop-16 (0-10 μM)for 24 hrs. Cell viability was measured using the MTT[3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] cellsurvival and proliferation kit (Millipore, Inc., Billerica, Mass.)according to manufacturer's instructions.

Western Blotting

Cell lysates or pulldowns were western blotted using routine laboratoryprocedures. Anti-RhoA, -Rac (1,2,3), -Cdc42, -p-PAK^(Thr423) antibodieswere from Cell Signaling Technology, Inc., Danvers, Mass. Anti Vav2 wasfrom Zymed, San Francisco, Calif.; and Rac3, PAK1, Tiam-1, and Trioantibodies were from Santa Cruz Biotechnology, Inc., Santa Cruz, Calif.

Precipitation of Activated GEFs with Recombinant Mutant Rac1 (G15A)Protein

MDA-MB-435 cells in culture media were lysed in 1% Triton X-100, 20 mMHEPES, pH 7.4, 150 mM NaCl, 5 mM MgCl₂, and protease inhibitors andprocessed. Equal amounts of protein from clarified lysates wereincubated for 1 h at 4° C. with glutathione agarose beads conjugated toGST-Rac1(G15A) nucleotide free mutant (Cell Biolabs, San Diego, Calif.),that were pre-incubated (for 1 h) with vehicle or 2 or 4 mM EHop-016.The beads were washed and the lysates and pulldowns immunoblotted forTiam-1, Trio, or Vav2.

Interaction of Tiam-1DH/PH Domain with Rac1(G15A)

His-tagged Tiam-1 DH-PH pET construct was transformed into Rosetta DE3E. Coli cells and clarified lysates purified by batch affinitychromatography using His-Select Nickel Affinity Gel (Sigma). Tiam-1 waseluted with 300 mM Imidazole and separated by a FPLC size exclusionSuperdex 200 column. Purity of Tiam-1 fraction at 1.7 mg/ml was observedto be >95% by SDS-PAGE. GST-Rac1(G15A) glutathione agarose orglutathione agarose beads alone were pre-incubated with varyingconcentrations of EHop-016 or NSC-23766 for 1 h in lysis buffer (1%Igepal, 20 mM HEPES, 150 mM NaCl, 5 mM MgCl₂, pH 7.5). PurifiedHis-Tiam-1 DH/PH domain was added at a concentration of 2:1 Rac1(G15A):Tiam-1, and incubated for another hr at 4° C. Pulldowns were washed 3×in 1% Igepal buffer and 1× in HEPES buffer and western blotted with ananti-His antibody to visualize His-Tiam-1DH/PH domain protein.

Fluorescence Microscopy

Metastatic human cancer cell lines MDA-MB-435 and MDA-MB-231 (in DMEMwith 10% FBS) were treated with vehicle (0.1% DMSO) or EHop-016 at 2 and4 μM for 24 hrs. Cells were fixed, permeabilized, and stained withRhodamine phalloidin to visualize F-actin.

Cell Migration Assays

Quiescent MDA-MB-435 cells were treated with vehicle or varyingconcentrations of EHop-016 (0-5 μM) for 24 hrs. Exactly 2×10⁵ cells wereplaced on the top well of Transwell chambers (Corning Life Sciences,Lowell, Mass.) with media containing 10% FBS in the bottom well. Thenumber of cells that migrated to the underside of the membrane following4 h incubation was quantified for each treatment.

Results

Synthesis of EHop-016

The inhibition of Rac1 activity has been proposed as a strategy for theprevention of cancer metastasis. However, the frequently utilized smallmolecule Rac1 inhibitor NSC23766 only has a moderate biological effecton the highly metastatic cancer cell line MDA-MB-435, even at highconcentrations; thus, illustrating the need for more potent andeffective inhibitors. Our group recently reported the synthesis of novelNSC23766 derivatives that showed increased Rac1 inhibitory activity inMDA-MB-435 cells with a considerable reduction of cell functionsregulated by Rac. Further research lead to the identification ofEHop-016, which was synthesized according to the procedure described inFIG. 1(A).

Briefly, 2,4-dichloropyrimidine (1) was reacted with3-amino-9-ethylcarbazole (2) by heating in iso-propanol in the presenceof N,N-diisopropylethylamine. Separation of the regioisomers providedthe pure 4-substituted pyrimidine derivative (3), which was reacted with4-(3-Aminopropyl)morpholine (4) in sec-butanol under microwave heatingin the presence of N,N-diisopropylethylamine to provide EHop-016 (5).

Molecular Docking of EHop-016

NSC23766 was designed to prevent the activation of Rac1 by binding tothe region where several GEFs interact with Rac1, thus inhibiting itsactivation. This mode of action was recently confirmed via analysis ofthe crystal structure of Rac1 with NSC23766. For the design of novel,more potent inhibitors, we desired to improve the activity and maintainthe selectivity profile of NSC23766. Therefore, in order to obtain asimilar binding interaction for the new inhibitors, the core structuralfeatures of NSC23766 were maintained, including a central pyrimidinecore, an aromatic 4-substituent and an amino group containing2-substituent, both connected to the pyrimidine core via nitrogen atoms.Molecular docking, to study the binding interactions of EHop-016 withRac1, demonstrated that EHop-016 can bind to Rac1 in differentorientations into the cleft formed by amino acids Pro34, Thr35, Va136,Phe37, Asp38, Asn39, Trp56, Asp57, Thr58, Ala59, Tyr64, Leu67, Arg68,Leu70 and Ser71. Clustering of the docking results revealed that thelargest cluster (size=29) also had the docking conformation with thelowest energy (−7.91 kcal/mol), and the most favorable conformation isillustrated in FIG. 1B, together with the position of NSC23766 in thecrystal structure of the Rac1-NSC23766 complex. Whereas NSC23766 isstretched over the surface of Rac1, similar to other novel Rac1inhibitors that were recently identified (60), EHop-016 appears to favora bent conformation that binds to a deeper binding pocket similar to theN,N-diethylamino group of NSC23766. In its energetically most favorableconformation, the binding of EHop-016 is strengthened by two hydrogenbonding interactions with residues Asp38 and Asn39. Furthermore,EHop-016 has a close interaction with Trp56, which has been shown to becritical for binding of Rac to its GEFs. Two smaller clusters (sizes=13and 21), with lowest energy conformations of −7.61 and −7.41 kcal/molrespectively, dock into the same cleft albeit in a somewhat differentmode as shown in FIGS. 8A-8B. Based on the above molecular dockingresults, structural similarity to NSC23766, and the biological activityprofile described herein, it is reasonable to postulate that EHop-016also interferes with binding of Rac1 with its GEFs via binding to thethree-way junction site that contains the switch I, switch II, and βloops of the effector region of Rac that interacts with the dbl homology(DH) domain of Rac-GEFs. The crystal structures of a complex of thebinding domains of the homologous GEF Vav1 with Rac1 were previouslydescribed. Using this information, analysis of binding interactionsindicates that unlike NSC23766, EHop-016 interacts with several of theamino acid residues that form the putative binding pocket of Vav1 withRac1. More specifically, of the residues that are calculated to interactwith EHop-016, Vav1 interacts with residues Thr35, Va136 and Asn39 ofswitch I, and residues Ala59 and Tyr64 of switch II as shown in FIG.1(B). Therefore, it is suggested that EHop-016 binds tightly to keyamino acid residues of Rac1, potentially inhibiting interaction withVav.

EHop-016 is a Potent Inhibitor of Rac1

The potential of EHop-016 to inhibit Rac activity was determined in thehighly metastatic human cancer cell line MDA-MB-435 that was previouslyreported by our group to contain high endogenous Rac activity.MDA-MB-435 cells were treated for 24 h with varying concentrations ofEHop-016, and for comparison, NSC23766. Rac activity from cell lysateswas measured using the G-LISA Rac1 activation assay. The concentrationcurves in FIG. 2 demonstrate that EHop-016 inhibits Rac1 activity inMDA-MB-435 cells with an IC₅₀ of 1.1 μM, while the IC₅₀ for NSC23766 inthe same cell line was 95 μM. Thus, EHop-016 is approximately 100 timesmore potent than NSC23766 and 10-50 times more potent than othercurrently available Rac inhibitors.

Effect of EHop-016 on the Activity of Other Rho Family GTPases

In order to investigate the selectivity of EHop-016, we studied itseffect on other Rho family GTPases. Similar to Rac1, the Rac isoformRac3 that is also overexpressed in cancer cells was inhibited by 58% ata concentration of 1 μM EHop-016 as shown in FIG. 3(A). This is expectedsince Rac1 and Rac3 demonstrate significant structure similarity and areactivated by the same GEFs. EHop-016 did not affect the activity of theRac homolog Cdc42 at 2 or 4 μM but inhibited Cdc42 activity by 28% at 5μM and 74% at 10 μM (FIG. 3B). Therefore, EHop-016 may not bind thesimilar DH-interacting domain of Cdc42 as tightly as Rac but is able toinhibit Cdc42 activity at higher concentrations. In contrast, theactivity of the closely related Rho GTPase RhoA was increased by˜1.3-fold in response to 2, 4, 5, or 10 μM EHop-016. Increased Rhoactivity, when Rac is being inhibited by EHop-016, may be a compensatorymechanism. This is expected to increase Rho-regulated assembly ofF-actin into stress fibers and further inhibit Rac-mediated lamellipodiaformation and directed migration (1; 4; 10).

EHop-016 Inhibits the Association of Active Vav2 with a Rac1 (G15A)Mutant Fusion Protein

To investigate a mechanism for the inhibition of Rac by EHop-016, theactive Rac-GEFs in MDA-MB-435 cells were detected from pulldowns of aglutathione agarose conjugated GST fusion protein of a Rac1nucleotide-free mutant Rac1(G15A) that has a high affinity for activatedGEFs (66; 69). As shown in FIG. 4(A), in the highly metastaticMDA-MB-435 cell line with elevated Rac activity, Vav2 appears to be moreabundant compared to Tiam-1 and Trio. Even though equal amounts of totalprotein were used for western blotting, it is possible that thedifferences in GEF expression may reflect the relative affinities totheir specific antibodies. Moreover, in MDA-MB-435 cells growing in 10%serum, only active Vav2 was pulled down with the GST-Rac1 (G15A) and notTiam-1 or Trio as shown in FIG. 4A). We show that the association ofVav2 with Rac1(G15A) was inhibited by 4 μM EHop-016 to ˜50% compared tocontrols in a statistically significant manner (p<0.005) as shown inFIGS. 4(B) and 4(C). Since, we did not detect association of Tiam-1 withRac1(G15A) under our experimental conditions, the effect of EHop-016 onthe interaction of Rac1 and Tiam-1 could not be measured in vivo.

When Rac1(G15A) beads were incubated with a purified Tiam-1 DH/PHdomain, the active Tiam-1 was associated with the Rac1(G15A) beads.However, EHop-016 inhibited the interaction of Tiam-1 DH/PH domain withRac1(G15A) only at concentrations 40 μM. At 5 μM, EHop-016 did notaffect the interaction of Tiam-1 DH/PH domain with Rac1(G15A) beads. At50 μM EHop-016, there was a statistically significant 64% inhibition ofTiam-1/Rac1(G15A) interaction. In contrast, the parent compound NSC23766inhibited the Rac1(G15A) interaction with purified Tiam-1 DH/PH domainat both 5 and 50 μM as shown in FIGS. 4(D) and 4(E). Therefore, theconcentrations at which EHop-016 inhibited Tiam-1/Rac1 interaction was10 times higher than the physiological concentrations (2-4 μM) at whichEHop-016 inhibited Rac activity or the interaction of Rac1 (G15A) withVav2 in MDA-MB-435 cell lysates.

EHop-016 Reduces Rac-Regulated Cell Functions

Rac is a central regulator of lamellipodia and invadopodia that controldirected mesenchymal migration and invasion of cancer cells. Therefore,we investigated the effect of EHop-016 on Rac activity and lamellipodiaformation in the low metastatic breast cancer cell line MDA-MB-231 andthe high metastatic MDA-MB-435 cells. As shown in FIG. 5(A), for equalamounts of total protein, Rac expression and activity in MDA-MB-231cells were less when compared to MDA-MB-435 cells. EHop-016 at 2 μM(double the IC₅₀ for Rac inhibition) inhibited the Rac activity ofMDA-MB-435 cells by 79% and by 93% at 4 μM. The Rac activity ofMDA-MB-231 cells was also inhibited by EHop-016 with an IC₅₀ of ˜3 μM asshown in FIG. 5(A). Therefore, EHop-016 is more efficient at inhibitingMDA-MB-435 cells with elevated Rac activity. Lamellipodia extension wasdetermined in cells stained with Rhodamine phalloidin to localizeF-actin. Treatment for 24 h with EHop-016 at 2 and 4 μM inhibitedlamellipodia formation in both MDA-MB-231 and MDA-MB-435 cells. Resultsshow that close to 100% of control cells demonstrated lamellipodia andmembrane ruffles. Following EHop-016 treatment, at 4 μM in MDA-MB-231cells and both 2 and 4 μM in MDA-MB-MB-435 cells, only ˜30% of the cellsextended lamellipodia. Therefore, EHop-016 at concentrations thatinhibit Rac activity significantly inhibited lamellipodia extension to asimilar extent, indicating a direct regulatory effect of Rac activity onlamellipodia extension as shown in FIG. 5(B). However, EHop-016 did notaffect the extension of filopodia that are regulated by Cdc42. Thesedata indicate a specific role for EHop-016 in inhibiting Rac-directedactin structures.

The effect of EHop-016 on Rac action was also determined by analyzingthe activity of the Rac downstream effector PAK that regulatesRac-mediated lamellipodia extension and directed cell migration. Bywestern blotting with a specific antibody to the kinase active form ofPAK1 with a phospho-Thr 423, we show that EHop-016 dramatically inhibitsPAK activity at both 2 and 4 μM as shown in FIG. 6(A). Since Rac/PAKsignaling regulates directed cell migration, the effect of EHop-016 at0-5 μM was determined on migration of MDA-MB-435 cells by using aTranswell assay. As shown in FIG. 6(B), treatment with 2 and 5 μMEHop-016 reduced directed cell migration by ˜60% at concentrations thatreduced Rac activity by ˜80% as shown in FIG. 2 and FIGS. 4A-4E. It ispossible that Rac may not be the only regulator of cell migration inthis highly invasive cancer cell line. Since equal cell numbers wereplaced on the top wells of the Transwell chambers and the migrationassays were conducted for only four hours, we do not expect effects ofreduced cell viability or cell death to affect data interpretation.

These experiments were conducted in culture media containing 10% FBS torecapitulate the endogenous cellular environment. Since serum containsactivators of Rac, Rho, and Cdc42, this experimental design does notallow for the analysis of Rac regulation in the absence of othercytoskeletal regulators.

Effect of EHop-016 on Cell Viability

We tested the viability of MDA-MB-231 and MDA-MB-435 cancer cells, andMCF10A transformed mammary epithelial cells in the presence of EHop-016.EHop-016 at the IC₅₀ of 1 or 2 μM did not significantly affect mammaryepithelial or cancer cell viability as shown in FIG. 7. At 5 μM,EHop-016 decreased MDA-MB-435 and MCF-10A cell number by ˜30% comparedto controls; however, this was only a trend. In these cell lines, cellviability was significantly decreased to ˜50% at 10 μM, while all threecell lines demonstrated a significant reduction in cell viability at 25μM. Therefore, further inhibition of Rac and Cdc42 activity by EHop-016at concentrations above 5 μM may lead to additional inhibition of Racand Cdc42-mediated effects on cell cycle progression and growth.

Discussion

Herein, we describe the synthesis and characterization of a potent andspecific small molecule inhibitor of Rac, a key signaling protein thatregulates cancer progression and metastasis. Recent studies havedemonstrated that inhibition of Rac activity, and thus cancer cellinvasion, is a viable strategy for the treatment of breast cancermetastasis. Current small molecule inhibitors of Rac activity such asNSC23766 and EHT 1864 are effective at high concentrations,approximately 50-100 μM for NSC23766 and 10 μM for EHT 1864, and theinhibitory efficiency appears to be dependent on cell type. As we havepreviously shown, high metastatic variants of MDA-MB-435 demonstratehigh endogenous Rac activity, without changes in Rac expression.Therefore high Rac-GEF activity may explain the moderate biologicalactivity of NSC23766 in metastatic cancer cells.

Our objective was to utilize the structure of NSC23766 as a lead for thedevelopment of a more potent inhibitor that similar to NSC23766, bindsspecifically into a surface groove of Rac1 known to be critical for GEFinteraction. Docking studies demonstrated that EHop-016 binds to theeffector domain of Rac1 with a deeper interaction in the switch 1 andswitch II regions than NSC23766. Therefore, EHop-016 has the potentialto block Rac-GEFs distinct from the action of NSC23766. Moreover,EHop-016 was found to be a ˜100-fold more potent inhibitor of Racactivity than NSC23766 in the metastatic MDA-MB-435 cancer cell line.EHop-016 is also a 10-50-fold more effective inhibitor of Rac1 thanrecently reported NSC23766 derivatives. Our data demonstrate thatEHop-016 is specific for Rac isoforms Rac1 and Rac3. Cdc42, a closehomolog that is not activated by the Rac-GEFs Tiam-1, Trio, or P-Rex,but is activated by Vav2 is inhibited by EHop-016 at higherconcentrations. Therefore, EHop-016 may be specific for Rac at lowerphysiologically relevant concentrations when Rac1 is the preferredpartner of Vav2. Because of its structural similarity to NSC23766, aswell as its biological activity profile and molecular docking findings,it is proposed that EHop-016 also interacts with the effector region ofRac, where it inhibits the binding of GEFs. In fact, we found thatEHop-016 at 50 mM, a concentration much higher than its physiologicallyeffective 2 mM concentration, may impede the interaction of activeTiam-1 with Rac1(G15A) in vitro. However, the Tiam-1 specific inhibitorNSC23766 blocked the interaction of Tiam-1 and Rac1(G15A) to a similarextent at a lower (5 mM) concentration compared to EHop-016; thus,indicating that EHop-016 is not specific to Tiam-1. Similarly, EHop-016is probably not specific to Trio, a GEF that shares the amino acidresidue Trp 56 in Rac1 with Tiam-1, as a critical determinant for theiractivity.

The significant finding that EHop-016 inhibits the interaction of Vav2with Rac1 at physiologically relevant concentrations is central to thefurther development of this compound as an inhibitor of cancermalignancy. Both Vav2 and Tiam1 have been implicated in Rac-mediatedtransformation and invasion/metastasis. To our knowledge, no specificinhibitors for Vav2/Rac interaction have been described. Using anestablished method for detecting active Rac-GEFs from cell lysates, wefound that Vav2, and not Tiam-1 or Trio, was active in the MDA-MB-435cell line in culture. It is possible that low active levels of Tiam-1and Trio were not detected due to the limited sensitivity ofimmunoblotting. Another alternative is that the serum in culture mediaactivated Vav2 preferentially and future studies will be conducted inquiescent cells following epidermal growth factor receptor (EGFR)/humanepidermal growth factor receptor 2 (HER2) stimulation, which is expectedto activate both Vav2 and Tiam-1.

Vav2 has been shown to act as an exchange factor for Rac, Rho, and Cdc42in vitro. The increased Rho activity following EHop-016 treatment may bedue to a complex biological response to Rac inhibition. Studies haveshown that Rac activation can inhibit Rho activity and vice versa.Indeed, disparate roles have been proposed for Vav2 activated Rho andRac in mammary epithelial cells; thus, suggesting that EHop-016 mediatedinhibition of Rac/Vav2 interaction may result in enhanced Vav2availability for Rho activation.

Herein, we show that cell viability is not affected by EHop-016 at thelow concentrations that specifically inhibit Rac and PAK activities andactin cytoskeletal changes. Racs and Cdc42 have been shown to affectcell survival via a number of signaling pathways includingphosphoinositide 3-kinase (PI3-K), nuclear factor kB (Nf-κB), and Junkinase (JNK)/p38 mitogen activated protein kinases (MAPK). Accordingly,cell viability is affected at higher concentrations (5 mM), whenEHop-016 inhibits both Rac and Cdc42 activities. In preliminary studies,we have tested the effect of oral and intraperitoneal administration ofEHop-016 to athymic nude mice at 1 mg/kg body weight once a week for 9weeks and found no change in body weight or gross indications oftoxicity as shown in FIG. 10.

Results show that EHop-016 reduced lamellipodia and directed cellmigration by 60-70% at concentrations that do not affect cell viabilitybut inhibit Rac activity by ˜80%. At a concentration of 2 μM, EHop-016exerted a dramatic reduction in lamellipodia formation without affectingCdc42-induced filopodia extension. The concentration at which EHop-016inhibited lamellipodia extension and cell migration is approximately10-50 fold less than the reported concentrations of NSC23766 and EHT1864required to inhibit lamellipodia extension and cell migration. Thisresult is similar to our previous report of decreased MDA-MB-435 cellmigration in the presence of other NSC23766 derivatives that inhibit Racactivity. Even though a single report demonstrated increased migrationof MDA-MB-435 and MDA-MB-231 metastatic breast cancer cells following50-100 μM NSC23766, in our hands, treatment with 100 μM NSC23766 for 24h resulted in a 75% decrease in MDA-MB-435 cell migration (data notshown). Moreover, there are several reports of decreased cell migrationin metastatic breast cancer cells following direct inhibition of Rac1and Rac3 by expression of dominant negative Rac and siRNA knockdown ofRac expression, or by indirect inhibition of Rac by blocking upstreameffector activity.

In conclusion, we have shown that EHop-016 is an effective Rac-specificinhibitor at low micromolar concentrations. EHop-016 substantiallyinhibits Vav2 interaction with Rac, Rac-activated PAK1, lamellipodiaformation, and cell migration. At concentrations above 5 μM, EHop-016also inhibits Cdc42 activity and cell viability. Since Rac/PAK activityis central to cancer cell migration and invasion, EHop-016 appears to bea promising candidate for further development as a pharmacologicalinhibitor of Rac activity in metastatic cancer cells. In addition,EHop-016 could prove to be a valuable, more potent probe for the studyof Rac-regulated cellular processes.

Although the invention has been described in conjunction with specificembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, the invention is intended to embrace all ofthe alternatives and variations that fall within the spirit and scope ofthe appended claims.

We claim:
 1. A method of inhibiting human cancer cell metastasis comprising administering N4-(9-ethyl-9H-carbazol-3-yl)-N2-(3-morpholin-4-yl-propyl)-pyrimidine-2,4-diamine, having the following chemical structure:

to a patient.
 2. The method of claim 1, wherein the cancer cell is a breast cancer cell.
 3. A method of inhibiting association of VAV with RAC on a human cancer cell comprising subjecting said human cancer cell to a compound N4-(9-ethyl-9H-carbazol-3-yl)-N2-(3-morpholin-4-yl-propyl)-pyrimidine-2,4-diamine, having the following chemical structure:


4. The method of claim 3, wherein said human cancer cell is of the cell lines MDA-MB-231 or MDA-MB-435. 